Physics/Chem: Why the sign difference in Thermodynamics?

In summary, the reason why we consider work done on the system as positive in chemistry and negative in physics is because physics always works from the system's point of view. This means that when the system loses energy, as in the case of pushing pistons or turning wheels, it is considered negative, while putting energy into the system is considered positive. This also applies to heat, where heating the system is positive and cooling the system is negative. Any discrepancies in this convention may be due to different systems being considered or different potential functions being used.
  • #1
ishan001
2
0
I have been wondering why we take work done on the system +ve in chemistry while -ve in physics. Similar difference exist for many other quantities. Can you explain why the difference exists?
 
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  • #2
There is no difference unless the Chemists are erring.

Physics always works from the system's point of view. The system is normally your gas or solution or what have you. Work done by the system pushing pistons, swirling wheels counts negative, because the system loses energy. And you can drive stuff with it. When you push the piston or turn the wheel from the outside then you put energy into the system so it counts positive.
Same for heat, heating the system positive cooling the system negative.

The reasons why things may disagree can be:
- You are talking about different systems
- Canonical transforms. If you use a different potential then the formulas look very similar, but are in fact different.
 

1. Why are there two different signs used in thermodynamics?

The two different signs used in thermodynamics refer to the different conventions of representing energy flow and work. In thermodynamics, energy flow is represented as positive when it flows into the system and negative when it flows out of the system. On the other hand, work is represented as positive when it is done on the system and negative when it is done by the system.

2. How do I know when to use a positive or negative sign in thermodynamics?

To determine when to use a positive or negative sign in thermodynamics, you need to consider the direction of energy flow and work. If energy flows into the system, a positive sign should be used, and if energy flows out of the system, a negative sign should be used. Similarly, if work is done on the system, a positive sign should be used, and if work is done by the system, a negative sign should be used.

3. Can the sign in thermodynamics change?

Yes, the sign in thermodynamics can change depending on the reference point or the direction of energy flow and work. The sign convention used in thermodynamics is arbitrary, and it is essential to be consistent within a problem or system. However, it is important to note that the change in sign does not change the physical meaning of the variables.

4. Are there any exceptions to the sign convention in thermodynamics?

Yes, there are a few exceptions to the sign convention in thermodynamics. One common exception is the sign convention used for heat transfer. In some cases, heat transfer is represented as positive when it flows out of the system, which is the opposite of the general convention. This is done to maintain consistency with other thermodynamic equations.

5. How does the sign convention in thermodynamics affect calculations?

The sign convention in thermodynamics does not affect the outcome of calculations as long as it is applied consistently. However, using the wrong sign convention can lead to incorrect results. It is essential to pay attention to the direction of energy flow and work and use the appropriate sign convention to ensure accurate calculations.

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